Liquid crystal display devices employ optical anisotropy and dielectric anisotropy of liquid crystal materials. As display mode, twisted nematic (TN) mode, super twisted nematic (STN) rode, dynamic scattering (DS) mode, guest-host (G-H) mode, and DAP mode are known. Further, as driving mode of the devices, static driving mode, time division driving mode, active matrix driving mode, and dual frequency driving mode are know,. While the properties of liquid crystal materials used for such liquid crystal display devices are different depending on their applications, it is required to any liquid crystal material that the material is stable against external environmental factors such as moisture, air, heat, and light, that the material exhibits a liquid crystal phase at a range of temperatures as wide as possible with room temperature being at its center, that the material has a low viscosity, and that the material is low in its driving voltage. Besides, liquid crystal materials used for liquid crystal display devices are usually composed of several kinds or 10-odd kinds of liquid crystalline compounds in order to obtain optimum dielectric anisotropy (.DELTA..epsilon.) or optical anisotropy (.DELTA.n) required to each display device. Accordingly, the miscibility with other liquid crystal compounds, miscibility at low temperatures is desired for liquid crystal materials from the recent needs of being used under several environments in particular.
On the other hand, active matrix mode, particularly TFT (thin film transistor) mode is popularly adopted recent years as a display mode for television or viewfinder from the aspect of display performances such as contrast, display capacity, and response time. Also, STN mode which is simple in manufacturing process and less expensive in production cost while having a large display capacity is largely adopted as display for personal computers and others.
As the trend in recent years, development in this field is being advanced with miniaturization of liquid crystal display devices or their downsizing to portable dimensions being centered, as typified by televisions which are characterized in that they are small-sized, light weight, and portable, and by notebook type personal computers. From the aspect of materials, development is being carried out with liquid crystalline compounds and liquid crystal composition having a low driving voltage, that is, a low threshold voltage as its center from the aspect of withstand voltage of IC.
It is known that threshold voltage can be expressed by the following equation (H. J. Deuling et al., Mol. Cryst. Liq. Cryst., 27 (1975) 81): EQU V.sub.th =.pi.(K/.epsilon.0.DELTA..epsilon.).sup.1/2
wherein K is an elastic constant and .epsilon.0 is a dielectric constant in vacuum. From the equation mentioned above, it is a common way to use a material having a large dielectric anisotropy (.DELTA..epsilon.) in order to reduce the threshold voltage, and thus, development of compounds having a large value of dielectric anisotropy is being actively carried out.
On the other hand, in liquid crystalline compounds having fluorine atom as substituent, it is known to be effective to increase the number of substitution on the molecule with fluorine atom for increasing dielectric anisotropy. However, the number of substitution on the molecule of compounds with fluorine atom has a proportional relationship with viscosity, and it is also empirically known among the person skilled in the art that when the number of substitution with fluorine atom is increased, mesomorphic range will be narrowed. Accordingly, it is considered to be difficult to increase only dielectric anisotropy while suppressing the raising of viscosity and narrowing of mesomorphic range.
As examples of conventional liquid crystalline compounds substituted with multiple number of fluorine atoms, the followings are disclosed: ##STR2##
Any of compounds (a) and (b-1) to (b-4) has several fluorine atoms at the terminal of the molecule. The value of dielectric anisotropy of the compounds are compared and arranged in the order of the value as follows:
(b-3)&gt;&gt;(b-1), (b-2), (b-4)&gt;(a)
Compound (b-3) has an ester bond as bonding group between molecular skeletons. It is considered that the large polarization of an ester carbonyl group effectively contributes to the dielectric constant of major axis of compound molecule and thus the compound exhibit an extremely large dielectric anisotropy. On the other hand, however, (b-3) can not be said to be preferable compound in the aspect of response speed from the fact that the compound has a remarkably high viscosity than (a), (b-1), (b-2), and (b-4). With respect to compound (c), it can not be said to be the compound which satisfies the requirements since it has a viscosity as high as (b-3) while (c) has a comparatively high dielectric anisotropy.
In this connection, compounds having --CF.sub.2 O-- at a terminal of the molecule have already been disclosed in patent publications as the compounds having a partial structure of --CF.sub.2 O-- in the molecule in liquid crystalline compounds. However, as the compounds having a partial structure which crosslinked two benzene rings as a bonding group in the structure of the compound, only compounds (d) and (e) shown above have been disclosed in Japanese Patent Application Laid-open Nos. 2-289529 and 5-112778, respectively. Besides, whereas the structural formula of the compounds have been described, physical data of the compounds and specific value of physical properties for evaluating the utility as liquid crystalline compound have not been disclosed in the publications. Accordingly, the properties of liquid crystalline compounds having a partial structure in which two benzene rings are bonded with --CF.sub.2 O-- group are not known at all. The present inventors had presumed that the dielectric constant in the direction of major axis of the molecule is offset in compound (d) since the polarization of --CF.sub.2 O-- group which is a bonding group in the molecule is arranged toward the direction opposite to the polarization of fluorine atom at the terminal of the molecule, and thus a large dielctric anisotropy can not be expected. Even with respect to compound (e), it was considered that a high clearing point can not be expected since it is a bicyclic compound in its skeltal structure while a comparatively large dielectric anisotropy can be expected from the contribution of polarization of --CF.sub.2 O-- group. As explained above, it was the actual situation that the effect of --CF.sub.2 O-- bonding group as well as the effect of compounds (d) and (e), particularly the knowledge on the relationship between dielectric anisotropy and viscosity were not known at all.